In the case of driving a synchronous motor having coils of a plurality of phases, it is conventionally important to optimize so-called conduction timing of flowing a motor current with suitable timing to a motor rotor and applying a voltage to a coil terminal. To detect a reference for this conduction timing, there are various systems, such as a system of detecting a back electromotive voltage, a system of detecting a zero-cross current phase, and the like.
For example, in so-called sensorless driving of controlling and driving a motor without using a motor rotor position sensor, there is a system of, when bringing a motor coil into conduction, detecting a back electromotive voltage generated in the motor coil by rotation of the motor, from a motor coil terminal.
In addition, in a driving device shown in Japanese Patent Laying-Open No. 5-236789 (Patent Literature 1), a system of detecting a motor voltage phase at the time of motor current zero crossing, detecting a motor current phase based on this voltage phase, and calculating a voltage command or a frequency command such that this motor current phase is a desired current phase is shown.
FIG. 10 shows a configuration of a conventional typical motor control device.
Referring to FIG. 10, to drive a synchronous motor 100 having coils of a plurality of phases (three phases) in a stator and a permanent magnet in a rotor, an inverter device is formed of an inverter 150, a converter circuit 130, an AC power supply 160, a coil 170, a current sensor 180, and a controller 110. It is noted that, in this example, AC power supply 160 shall be 200V and 50 Hz.
Synchronous motor 100 is driven by inverter 150, and inverter 150 is supplied by converter circuit 130 with a DC voltage obtained by converting AC power supply 160 into a direct current.
Specifically, converter circuit 130 includes a diode full wave rectifying circuit 120 formed of diodes 122 to 128 and a smoothing capacitor 140 across buses, and the capacity of the smoothing capacitor is large enough to such a degree that a ripple in a DC voltage waveform can be suppressed.
An AC voltage of AC power supply 160 is converted by this converter circuit 130 into a DC voltage for supply to inverter 150.
Coil 170 is provided for the purpose of improving the power factor of the AC power supply supplied to converter circuit 130.
FIG. 11 is a drawing for explaining the relation between DC voltage waveform and U-phase motor current.
As shown in FIG. 11(a), when the capacity of smoothing capacitor 140 is large enough, the ripple in the DC voltage waveform is suppressed, and a constant DC voltage is supplied to inverter 150.
FIG. 11(b) is a drawing for explaining a U-phase motor current waveform detected by current sensor 180.
Since the ripple in the DC voltage waveform is suppressed and a constant DC voltage is supplied to inverter 150 as shown in FIG. 11(b), the U-phase motor current that drives synchronous motor 100 from inverter 150 is detected as a stable waveform of constant amplitude.
On the other hand, since a conventional configuration as described above is disadvantageous in that an improved input current waveform and a higher power factor are less likely to be achieved, Japanese Patent Laying-Open No. 2002-51589 (Patent Literature 2) proposes a system without using coil 170 but with a capacitor, having a small capacity of about 1/100 of the capacity of a conventional smoothing capacitor, provided across buses of an inverter to intentionally cause a ripple of a frequency twice that of the power supply to occur in a DC voltage, thereby achieving an improved input current waveform and a higher power factor with a simple system.